1. Field of the Invention
The present invention relates to a semiconductor wafer comprising a monocrystalline substrate wafer consisting essentially of silicon which has a (111) surface orientation and a monocrystalline layer of AlzGa1-zN with 0≦z≦1 having a (0001) surface orientation.
2. Description of the Related Art
Monocrystalline gallium nitride (GaN) is of increasing importance as a substrate for producing light emitting diodes (LEDs) and field effect transistors (FETs) for high power and high frequency applications. The surface area of the substrate is a key issue for the productivity of the manufacturing process and therefore for the cost of an LED or FET. As monocrystalline silicon substrates with diameters of up to 300 mm or even up to 450 mm are currently available with high crystal and surface quality, efforts are being made to use monocrystalline silicon as a substrate for the epitaxial growth of GaN. However, due to the large lattice mismatch of 17% between GaN(0001) and Si(111) and the large difference in the thermal expansion coefficients (TEC) of the two materials, high quality GaN layers cannot be grown directly on silicon (Si).
For this reason, many types of intermediate or buffer layers have been proposed in order to increase the quality of the epitaxially grown GaN layer.
For example, M. Moram et al., J. CRYSTAL GROWTH 308 (2007), 302-308 teach the use of an epitaxial (111) oriented scandium nitride (ScN) buffer layer as a basis for GaN epitaxy. The ScN layers with a thickness ranging from 50 to 450 nm were grown on n-type Si(111) wafers using gas-source molecular beam epitaxy (GS-MBE) with ammonia as the nitrogen precursor. GaN layers were grown on the ScN/Si(111) templates using a Thomas Swan close-coupled showerhead MOVPE reactor operating at 100 Torr. In preparation for GaN growth, each template used was heated to the growth temperature at a rate of 1 K/s under a flow of 10 slm (standard liter per minute) ammonia (NH3) and 10 slm hydrogen (H2) in order to remove any native oxide from the surface of the ScN layers. The ScN buffer layer was rough and full of defects. The GaN grew on the ScN surface in the form of dislocation-free islands. By varying the GaN growth temperature, the GaN islands could be caused to coalesce in order to yield a smooth GaN film, but at the same time the density of threading dislocations on the surface of the GaN layer increased considerably up to 5×109 cm−2.
W. C. Lee et al., J. CRYSTAL GROWTH 311 (2009), 2006-2009 disclose single-crystalline scandium oxide (Sc2O3) as a template for GaN epitaxy. Sc2O3 films were deposited using e-beam evaporation from a high-purity powder-packed-sintered Sc2O3 oxide source. A few monolayers of aluminium (Al) were deposited on the Sc2O3 surface using molecular beam epitaxy (MBE), followed by the exposure to nitrogen plasma for nitridation in order to form a thin aluminium nitride (AlN) layer. GaN was deposited on top of this layer. The substrate temperature for growing AlN was at about 645° C. in the beginning and was raised to 720° C., which was also used for the rest of the GaN growth. The method results in a high defect density in the Sc2O3 buffer and a limited GaN layer quality. Dislocations occurred in a density in the range of 1010 cm−3. The dislocations propagated throughout the layer starting from the Sc2O3/Si interface.